This invention relates to a fail-safe gate valve for controlling the flow of fluids through a flowline and a method for operating the gate valve. More specifically, the invention relates to a fail-safe gate valve having a power actuator and a spring actuator located in separate housings with the housings disposed on opposite sides of the flowline.
The fail-safe gate valve of the present invention can be used anywhere there is a need for a valve with a fail-safe mechanism both subsea and on conventional land operations. The applications for the present invention include, but are not limited to Christmas trees, blowout preventer (BOP) stack valves, production valves and manifold valves.
A typical gate valve includes a valve body having a flowway through which fluids pass and a valve chamber intersecting the flowway. A valve element or gate is disposed within the valve chamber for reciprocation across the flowway wherein flow is permitted when a hole located in the gate is positioned within the flowway. Flow is blocked when the hole in the gate is shifted from the flowway. The gate is reciprocated within the valve chamber by a power actuator which may be hydraulic, pneumatic or electrical for opening or closing the gate valve.
In oilfield operations having long unattended service, it is desirable for gate valves to have fail-safe operators. A fail-safe operator returns a gate valve to a safe (normally closed) position when a loss of hydraulic, pneumatic or electrical power occurs. The fail-safe operator contains a spring actuator which ordinarily is disposed within the same housing containing the power actuator.
Disposing the spring actuator and the power actuator in the same housing makes the actuator large and can cause spacing and handling problems when installing gate valves. Some of these problems occur because of the additional housing length required when the spring actuator is included. For large bore Christmas trees, the actuator housing may be so large that it extends beyond the guide frame. In such cases, the probability of damage to the actuators is increased, the Christmas trees must be oriented to prevent interference between actuators caused by closer spacing in template systems, and the Christmas trees will be poorly balanced for handling. Spacing is also becoming more of a problem with increased usage of remote operated vehicles (ROVs) in place of divers to perform work on subsea systems.
The weight and size of the actuators increase as the flow way size of the gate valve increases. The increased weight and size of the actuators displaces the center of gravity of a Christmas tree. The offset frequently is of such magnitude that large valve systems cannot be safely handled. For example, Christmas trees with large bore block valves cannot be landed onto a wellhead without the addition of counterweights. Of course, increased weight of the counterweights further aggravates the handling problem.
Reducing the size and length of the actuators in fail-safe gate valves has been a goal of designers in the oil industry for a number of years. Some have attempted to reduce the length of the housing by changing the type of spring used, such as substituting conical compression springs or belleville washers. While reducing the length of the housing, nonstandard spring types are expensive to develop and manufacture, especially in small quantities.
I have determined that a solution to the above problem is to separate the power actuator from the spring actuator. The power actuator and the spring actuator are each contained in separate housings with the housings disposed on opposite sides of the gate valve. By placing the two actuators in different housings, the length of each housing is considerably shortened. Furthermore, by placing the housings on opposite sides of the gate valve, the offset in the center of gravity has been reduced.
With my novel design, I have further discovered that a double acting override can be incorporated into my spring actuator and the gate valve may be operated without the use of the power actuator. This would result in a reduction of input force or torque to operate the gate valve, since the springs will not be compressed during operation of the valve with the override.